HR: 1330h
AN: V32C-1047    [PDF]
TI: Recently Measured Helium Diffusion Rate for Zircon Suggests Inconsistency With U-Pb Age for Fenton Hill Granodiorite
AU: * Humphreys, D R
EM: drhumph@swcp.com
AF: Institute for Creation Research, P.O. Box 2667, El Cajon, CA 92021 United States
AU: Baumgardner, J R
EM: baumgardner@lanl.gov
AF: Los Alamos National Laboratory, MS B216, Los Alamos, NM 87545 United States
AU: Snelling, A A
EM: aasnelling@ozemail.com.au
AF: Geo-Research Pty Ltd, P.O. Box 1208, Springwood, Qld 4127 Australia
AU: Austin, S A
EM: saustin@icr.edu
AF: Institute for Creation Research, P.O. Box 2667, El Cajon, CA 92021 United States
AB: Gentry $et$ $al.$ ($GRL,$ $9,$ 1129-1130, 1982) reported high levels of helium retention in zircons extracted from granodiorite core obtained from the 4300 m deep borehole GT-2 at Fenton Hill, NM, drilled as part of a Los Alamos geothermal research project. These investigators measured, for example, 27% retention at a depth of 2170 m and temperature of 151$\deg$ C and 17% retention at a depth of 2900 m and temperature of 197$\deg$ C. The U-Pb concordia age determination for this rock was 1500$\pm$20 Ma at a depth of 2900 m. They inferred the total helium production from the radiogenic lead in the zircons. These surprisingly high levels of radiogenic helium retention motivated us to determine the helium diffusion rate in zircon extracted from this same granodiorite. We contracted with a high precision laboratory for these determinations. We used the same zircon size as Gentry $et$ $al.$ (50-75 $\mu$m) and obtained diffusion rate measurements over the temperature range 50-500$\deg$ C. At 200$\deg$ C, for example, the measured diffusion coefficient was 1.42 x 10$^{-17}$ cm$^{2}$/s. We also confirmed the high helium retention, finding 1320 nmol/cc (42% retention) in zircons from a depth of 1490 m and a temperature of 125$\deg$ C. By contrast, the surrounding biotite had only 7 nmol/cc. Our determinations gave a U-Pb concordia age of 1440$\pm$2 Ma. Astonishingly, for the current borehole temperatures and measured diffusion coefficients, we find the zircons could have retained the observed levels of helium only if the time span were less than about 10 ka. Today, most of the rock column is well above the calculated zircon helium closure temperature of 128$\deg$ C. According to published Los Alamos thermal models, temperatures in the region sampled by the borehole would have been much higher for the past 2 Ma, cooling down to today's levels only during the last 0.1 Ma. Even with present temperatures, our diffusion rates (which agree with published rates for other zircons) require retentions to be orders of magnitude lower than the observed levels to be consistent with the U-Pb age for these rocks.
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting